Non-emissive electrode for electron discharge device



Aug. 5, 1958 e. A. ESPERSEN 2,346,609

NQN-EMISS IVE ELECTRODE FOR ELECTRON DISCHARGE DEVICE Filed Feb. 8; 1954 \\\1 NT 3&1

9 9 9 9 9 9 9 99999 .J fiTITANIUM METAL GRID WIRES I -E+-. l 1 2 f I I -CATHODE CONTAINING ALKALINE EARTH METAL COMPOUND Inventor GEORGE AND/-75 W ESPERSE/V Agent United States Patent NON-EMISSIVE ELECTRODE FOR ELECTRON DISCHARGE DEVICE George Andrew Espersen, Dobbs Ferry, N. Y., assignor, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application February 8, 1954, Serial No. 408,892

5 Claims. (Cl. 313-107) This invention relates to non-emissive electrodes for use in electron discharge devices, and more particularly to those devices comprising a cathode containing an alkaline earth metal or compound thereof.

Modern electron discharge devices adapted for use in the higher frequency portions of the electromagnetic spectrum are generally characterized by close spacing between an incandescent cathode within the tube and an adjacent grid electrode. During operation of such tubes, the grid electrode tends to attain a substantially high elevated temperature as a result of direct heat radiation from the cathode, and thereupon proceeds to emit electrons as a primary source. This emission from the grid constitutes a great disadvantage since it introduces a number of formidable problems in connection with the design of the associated circuits. The situation is further complicated by the presence of a cathode in the tube containing an alkaline earth metal compound to enhance electron emission. Such cathodes are the well-known oxide-coated cathode, which consists of an indirectly-heated, nickel sleeve coated with a mixture of alkaline earth oxides, and the so-called dispenser cathode, which comprises an emission-enhancing composition containing an alkaline earth material enclosed by a porous refractory metal member as described, for example, in U. S. Patent #2,543,728. With these cathodes, there is the tendency for alkaline earth metal or oxides thereof to evaporate off the cathode and deposit on the adjacent grid electrode, which deposition has the eifect of markedly increasing the primary emission from that electrode.

Extensive investigation into this problem has resulted in the discovery of certain materials which inhibit or reduce the primary emission from a grid electrode. One of the most popular of these materials is gold, and the use of gold-plated grids to reduce primary grid emission is now utilized by most manufacturers. Such grids, however, suifer from the obvious drawback of high cost.

The chief object of the invention is to provide a nonemissive electrode for an electron discharge tube including a cathode containing an alkaline earth material which is relatively cheap to manufacture.

A further object of the invention is the provision of nonemissive electrodes for an electron discharge tube including a cathode containing alkaline earth material which exhibits reduced grid emission.

These and other objects of the invention will be best understood from the following description.

According to the invention, I have found that titanium metal when employed as a grid material will result in extremely low primary grid emission therefrom during operation. More particularly, I have found that by providing at least the surface of a grid electrode in a discharge tube containing an alkaline earth cathode with at least 90% by volume of titanium metal, the primary grid emission from that electrode is sharply reduced.

The non-emissive grid electrode of the invention may take one of three preferred forms. The first and simice plest involves the use of ordinary annealed, ductile, titanium metal wire as the grid material. This has the immediate advantage of eliminating any further manufacturing steps in the fabrication of the grid electrode, such as, for example, an electroplating step. The reason for employing annealed, ductile, titanium wire is to enable the fashioning of the grid electrode, for example, by winding around a pair of supporting posts, without fear of fracturing the wire.

A second embodiment of the invention contemplates the use of conventional grid wires of tungsten or molybdenum coated with a thin coating of titanium metal, of at least 0.0005 inch thick. This may be accomplished by means of ordinary electro-plating techniques, such as that described in Transactions of the Electro-Chemical Society, 1948, 93, 271-285.

The third embodiment contemplated involves the use of a grid wire constituted by an alloy of titanium with one of the metals tungsten, molybdenum or rhenium. In the latter case, the titanium should predominate over the other constituent, that is to say, the titanium should constitute at least 90% by volume of the alloy. In this embodiment, as in the first embodiment described, no coating is employed on the grid wire, thereby eliminating any further fabricating steps.

In accordance with a further feature of the invention, even superior results to that obtainable from the previous embodiments described above is obtained by providing a clean, highly-polished surface on the titanium, titanium-alloy, or titanium-coated grid wire. This may be accomplished by drawing the completed grid wire in an inert atmosphere, e. g., argon, helium or other rare gases.

In order to illustrate some of the advantageous results realized by means of the invention, there follows below the tabulated results of some of the experiments undertaken in reducing the invention to practice, compared to the results obtained with conventional grid constructions. In these experiments, a number of grid materials were placed in a conventional, high-frequency, triode-type of tube structure containing a dispenser cathode of the type described in U. S. Patent 2,700,000. A pulsed voltage was applied between the plate and cathode of the tube with a peak of 600 v., a pulse length of 100 microseconds and a duty cycle of 0.006, and the primary emission measured at different grid temperatures.

Grid Tem- Grid Emis- Grid Material perature in sion in Degrees, C. M. A.

As is evident from the foregoing data, a grid construction according to the invention afforded even better re sults, viz, lower emission, than a gold-plated grid. Further, the use of titanium is considerably cheaper than the use of a noble metal such as gold.

The grid of the invention has particular utility in tubes containing alkaline earth cathodes, because the latter tubes suifer most from the drawback of high grid emission due to the fact that evaporated products of the cathode deposit on the grid and reduce its work function.

The accompanying drawing is a cross-sectional view of a high frequency tn'ode discharge tube embodying one form of non-emissive' electrode in accordance with the lnvention.

In the drawing, an envelope 1 encloses a dispenser cathode 2 of the type described in the aforementioned U. S'.- Patent 2,700,000, comprising a porous tungsten member impregnated with -ba rium' aluminate, i'aier, an alkaline earth metal compound, and heated by a-heater filament 3. An anode 4 is located at'the other end E the envelope; A grid support; 5 supports grid wires 6 between the anode 4' and cathode 2-, which Wires 6' are-constituted by titanium metal in accordancewith theinvention; 7

While I have described my invention in connection with specific embodiments and applications, other modifications'th'ereof will be readily apparent fo those skilled in this art Without departingfromthe spirit and scope ofthe invention as d'efi'nedi'nthe appended claims.

l. A vacuumtube comprising a thermionic cathode containing a volatilizable alkaline eartlrmetal compound, and anon-emissive electrode located adjacent said ca'thode-in the path of alkaline earth metal compound particles emitted therefrom, saidelectrode consisting of annealed ductile substantially pure titaniummetal wire having a smooth; 'bright surface.

2. A vacuum tube comprising a thermionic cathode containing volatilizable alkaline earth metal compound;

and a non-emissive grid electrode located adjacent said i cathode in the path of alkaline earth metal compound particles emitted therefrom, said electrode comprising a refractory metal core and a coating of smooth, brig-ht, substantially pure titanium metal onsaid core. 7

3'. A vacuum tube comprising; a thermionic cathod'e containing a vol'atilizable alkaline earth metalcompound, and' a non-emissive electrode located adjacent: said cathode in the path of alkaline earth metal compound particles therefrom, saidelectrode consisting of an alloy of at least about by volume of titanium and not more than about 10% of one of the metals selected from the group consisting of tungsten, molybdenum and rhenium and having a smooth; bright surface.

4. A vacuum tube comprising a thermionic cathode containing avolatilizable alkaline earth metal compound, j

and a non-emissive electrode located adjacent said cathode in the path of alkaline-earth metal-compound particles from said cathode, said electrode having at leasta surface layer consisting of at least 90% by volumeof'titanium metal, and the balance principally a metal selected from the group consisting of tungsten, molybdenum and'rhev nium; and'having a smooth, bright surface.

5. A vacuum: tube comprising a thermionic cathode comprising a volatilizable electron emissive material, and a non-emissive metal electrode located adjacent. said cathode'inthe pathofparticles of said volatilizable' material therefrom,- said electro'deliaving a smooth; bright surface layer consisting of at least 90% by volume of titanium metal and the balance principally a metal selected from the group'con'sisting of tungsten; molybdenum and rhenium.

Van Beckum a Dec. 19; 1950 

1. A VACUUM TUBE COMPRISING A THERMIONIC CATHODE CONTAINING A VOLATILIZABLE ALKALINE EARTH METAL COMPOUND, AND A NON-EMISSIVE ELECTRODE LOCATED ADJACENT SAID CATHODE IN THE PATH OF ALKALINE EARTH METAL COMPOUND PARTICLES EMITTED THEREFROM, SAID ELECTRODE CONSISTING OF ANNEALED DUCTILE SUBSTANTIALLY PURE TITANIUM METAL WIRE HAVING A SMOOTH, BRIGHT SURFACE. 